1. Field of the Invention
This invention relates to an image formation apparatus capable of forming a synthetic image by superimposing image informations, using electrophotography or the like.
2. Description of the Related Art
In a conventional image formation apparatus using electrophotography, an electrophotographic photosensitive member, serving as an image-carrying member, is charged by a static charger, and light is applied to this photosensitive member in accordance with image information to form a latent image, and this latent image is developed by a developing device to form a toner image, and the formed toner image is transferred to a sheet or the like, thus forming the image.
On the other hand, with the advent of color images, there has been proposed an image formation apparatus of the tandem type in which a plurality of image-carrying members are provided, each of which is for effecting such an image-forming process, and color images, that is, a cyan image, a magenta image, a yellow image and preferably a black image, are formed on the respective image-carrying members, and these color images are transferred at respective transfer positions of the image-carrying members to a sheet in a superimposed manner, thereby forming a full-color image.
Such an image formation apparatus of the tandem type, because of the plurality of image-forming portions for forming the color images, is advantageous for a high-speed operation. What is important is to satisfactorily effect the registration in position of the color images formed by the different image-forming portions.
The reason is that a misregistration of the four color images, transferred to the sheet, results in a color drift or a variation in color tone.
FIG. 8 shows a conventional image formation apparatus using four photosensitive drums.
As shown in FIG. 8, the image formation apparatus comprises the photosensitive drums (photosensitive members) 51 for respectively forming toner images of four colors, for example, black (K), yellow (Y), magenta (M) and cyan (C), which drums serve as image-carrying members, and toner image-forming members 3 for forming the toner images on the surfaces of the photosensitive drums 51, respectively.
The toner image-forming member 3 comprises a charger 11 for charging the surface of the photosensitive drum 51 with electricity, an exposure device 12 for forming an electrostatic latent image, corresponding to specified color image data, on the charged photosensitive drum 51, a developing device 13 for developing the electrostatic latent image, a first transfer roller 21, which has an elastic material, e.g. an electrically-conductive urethane sponge, formed on a surface thereof, and urges an intermediate transfer belt 52 toward the photosensitive drum 51 so as to transfer the toner image, developed on the surface of the photosensitive drum 51, to the intermediate transfer belt 52, and a cleaner 14 for removing the residual toner remaining on the photosensitive drum 51 after the transfer of the toner image to the intermediate transfer belt 52.
The intermediate transfer belt 52, arranged along the plurality of photosensitive drums 51, are supported by a drive roller 20, a driven roller 22 and a tension roller 28, and is revolved in a direction of arrow A. An intermediate transfer belt cleaner 27 for removing the residual toner, remaining on the surface of the intermediate transfer belt 52, is provided adjacent to this belt 52. A color drift sensor 29 for detecting the amount of a color misregistration or drift of the toner images on the intermediate transfer belt 52 is provided adjacent to the intermediate transfer belt 52.
A paper cassette 15, storing printing paper 7, is provided at a lower portion of the apparatus, and the printing paper sheets 7 are fed one by one into a path of feed of the paper by a feed roller 16.
Resist rollers 17 for controlling the transfer timing for the printing paper 7, a second transfer roller 10 for transferring the color image, formed on the intermediate transfer belt 52, to the printing paper 7, and a fixing device 40 for fixing the transferred color image to the printing paper 7 by a fixing roller 18 and a pressure roller 19 are provided in the paper feed path.
In the image formation apparatus of this construction, for example, a latent image of a black color component of image information is first formed on the photosensitive drum 51 corresponding to the black color. This latent image is formed into a visible image, serving as a black toner image, by the developing device 13, and this black toner image is transferred onto the intermediate transfer belt 52 by the first transfer roller 21.
During this operation, a latent image of a cyan color component of the image information is formed on the photosensitive drum 51 corresponding to the cyan color, and this latent image is developed as a cyan toner image by the developing device 13, using a cyan toner. Then, the cyan toner image is transferred by the first transfer roller 21 onto the intermediate transfer belt 52, to which the black toner image has already been transferred, in superimposed relation to this black toner image.
Subsequently, a magenta toner image and a yellow toner image are formed in the same manner as described above, and thus the toner images of the four colors are superimposed together on the intermediate transfer belt 52.
On the other hand, the printing paper 7 is fed from the paper cassette 15 by the feed roller 16, and the toner images of the four colors are transferred at a time onto the printing paper 7, controlled in transfer timing by the resist rollers 17, by the second transfer roller 10. The thus transferred toner images are heated and fixed to the printing paper 7 by the fixing roller 18 and the pressure roller 19, so that a full-color image is formed on this printing paper 7.
The residual toner is removed from each of the photosensitive drums 51 by the cleaner 14 after the transfer of the toner image therefrom is finished.
In the above conventional construction, each transfer roller 21 is rotatably supported beneath the corresponding photosensitive drum 51, with the intermediate transfer belt 52 held therebetween. The outer diameter of the transfer roller 21, defined by the outer peripheral surface thereof, changes with the lapse of time, so that the force of pressing of the intermediate transfer belt 52 against the transfer roller 21 gradually decreases. Therefore, taking this into consideration, the force of pressing of the transfer roller 21 against the intermediate transfer belt 52 has beforehand been set to a little higher value.
And besides, the toner image is transferred from each photosensitive drum 51 to the intermediate transfer belt 52, and therefore the intermediate transfer belt 52 and the photosensitive drum 51 stand in contact with each other while maintaining the surface potential of the photosensitive drum 51.
Furthermore, the intermediate transfer belt 52 is pressed into the surface of each photosensitive drum 51 by the elastic force of the elastic material formed on the surface of the first transfer roller 21.
Furthermore, because of manufacturing errors such as a variation in the outer diameter and the eccentricity of the rotation axis, the peripheral speed of the photosensitive drum 51 deviates from the predetermined speed, so that the speed difference develops between the photosensitive drum 51 and the intermediate transfer belt 52.
Because of these factors, a dip or sag due to an electrical attracting force develops in the intermediate transfer belt 52 between any two adjacent photosensitive drums 51, and the belt distances between the adjacent photosensitive drums 51, that is, the belt distance between K and Y, the belt distance between Y and M and the belt distance between M and C, are varied, and this causes a deviation in the timing of transfer from each photosensitive drum 51 to the intermediate transfer belt 52.
As a result, a misregistration of the toner images, transferred to the intermediate transfer belt 52, occurs, and this appears as light and shade due to a color drift, which has invited a problem that the picture quality has been greatly degraded.
This problem has been a very big barrier to the formation of an image of a high picture quality with no color misregistration or drift in a digital-color image formation apparatus.
It is therefore an object of this invention to provide an image formation apparatus in which a dip or sag of an intermediate transfer belt between adjacent ones of a plurality of photosensitive members is reduced by decreasing a speed variation of the intermediate transfer belt due to a speed variation of the photosensitive members.
In order to achieve the above object, the image formation apparatus of the invention has a construction wherein each of transfer rollers, which are disposed below an intermediate transfer belt to press the intermediate transfer belt toward respective photosensitive drums, is resiliently provided at a position different from a point of contact between the associated photosensitive drum and the intermediate transfer belt, thereby adjusting the force of pressing of the intermediate transfer belt against the photosensitive drum.
The image formation apparatus according to another aspect of the invention has a construction wherein there is provided a device for removing or neutralizing an electric charge of each photosensitive drum or the intermediate transfer belt so as to reduce an electrostatic attracting force acting between the intermediate transfer belt and the photosensitive drum.
The image formation apparatus according to still another aspect of the invention has a construction wherein the peripheral speed of the intermediate transfer belt is set higher than the peripheral speeds of the photosensitive drums.
With those constructions, a frictional contact force between the intermediate transfer belt and each photosensitive drum is adjusted so that a slip can develop between the intermediate transfer belt and the photosensitive drum, and therefore a sag or dip is prevented from occurring in the intermediate transfer belt, thereby preventing a color misregistration or drift.